E-Mail A new study by ecologist André de Roos* shows that differences between juveniles and adults of the same species are crucial for the stability of complex ecological communities. The research, published in Proceedings of the National Academy of Sciences, represents a major advance in ecological modeling at a time when biodiversity is declining and species around the world are rapidly going extinct. Up to now, ecological models have focused exclusively on the interactions between species, ignoring the variations within them. The dragonflies, frogs, trout, and phytoplankton in a freshwater pond, for example, would be represented as nodes in a network, connected by edges that represent how each species feeds on the others.